CERAMICS AND CATALYSIS - Saint-Gobain

CERAMICS AND CATALYSIS

Ceramic technology is of crucial importance to the catalysis industry.

Dr. Ken J. Mills find important application. A growing application is

Manager of Business Development the heterogenization of homogeneous catalysts by

Saint-Gobain NorPro tethering the active entities to ceramic and other

3840 Fishcreek Road particles.

Stow, OH 44224-5400

[email protected] Some heterogeneous catalysts are solids, but do not

rely significantly on ceramic technology, i.e. no

It has been estimated that over $1.2 trillion in goods sintering process takes place. Such materials include

and materials are produced using catalytic activated carbon which forms the basis of many

technology in the United States -- 30% of GNP. The precious metal catalysts.

worldwide market for catalysts is currently estimated

at $12 billion, with the US portion at about 35%. The most efficient catalysts known are enzymes and

Overall, the market is growing at about 3.7% other biological entities -- these have recently been

annually, with a higher growth rate in environmental applied on an industrial scale. Porous ceramic

catalysis and in the rest of the world, outside North materials are of value in immobilizing these active

America, Western Europe and Japan (Table 1). species. Another recent development is the use of

photocatalytically active semi-conductor materials,

Material volumes are more difficult to estimate as such as titania, for waste stream purification under

catalyst costs range from less than $1/lb to more the influence of ultra-violet light.

than $50/lb. An additional characteristic of the

catalyst market is that most processes and catalyst Catalyst Carriers

formulations are proprietary and confidential to their Most catalysis takes place over metallic or metal

owners. compound surfaces. It is important to obtain a finely

dispersed layer of active material crystallites for good

Catalysts catalytic activity, and to use the costly metals more

Catalysts allow chemical reactions to take place at efficiently. The carrier allows this, and also adds

much faster rates or at much lower temperatures physical strength and enables special shapes to be

than in their absence. They are usually not made. Originally referred to as the “inert support,”

consumed during the reaction process, but do the catalyst carrier is now considered integral to the

become less effective over time. A homogeneous catalyst system and is developed in tandem with the

catalyst is dissolved in the reaction medium and catalyst. For example, catalytic selectivities for the

needs to be separated from the product stream after ethylene oxide (EO) production process have risen

reaction. A heterogeneous catalyst is a solid over from 60 - 65% in the 1960s to over 90% now, using

which the reactants pass and are converted to the the same basic silver/alpha-alumina catalyst. The

desired products. This is the area in which ceramics principal differences that have led to this large

increase in performance

Ta ble 1: Ca ta lyst Ma rke t Size ($Billion) are promotion systems

North W e st Re st of W orld and carrier

Am erica Europe Ja pa n W orld Tota l Grow th modifications. As each

Re fine ry 1.0 0.4 0.1 0.8 2.3 +2.0 - 2.5% percentage point

Che mica l 1.5 1.3 0.4 1.4 4.6 +2.5 - 3.0% improvement can be
Environm e nta l 1.7 1.6 1.2 0.6 5.1 +4.5% worth several million
Tota l 4.2 3.3 1.7 2.8 12.0 +3.7% dollars/year to a world
Grow th +3.0% +3.7% +3.2% +5.0%

2 ft. or more; impact resistance -- the particles may be
dropped this distance during fast loading; and
scale EO plant, the importance of the carrier is easily abrasion resistance -- especially important in fluid-
recognized. bed or moving bed reactor designs.

Physical Properties Size and Shape are Also Important
Internal surface area and porosity are perhaps the
most important physical attributes of the carrier, Shape affects the overall system pressure drop
because they allow active and selective catalysts to (Figure 2). However, low pressure drop shapes may
be produced. It is increasingly recognized, however, contain larger void spaces, which do not contribute
that the overall performance of the catalyst relies on to catalytic activity, and may reduce particle strength.
a wide range of physical properties being optimized Extrudates, typically 1 - 4 mm in diameter are still
for a particular reaction. used in many applications, but spheres and rings
(hollow cylinders) are rapidly gaining in importance.
Catalyst carriers are controlled porosity materials Especially in refining applications, extrudates with
where high internal surface areas are obtained by cloverleaf cross sections, known as trilobes or
incorporating into the matrix large volumes of quadrilobes, are critical to incorporating maximum
interconnecting pores. Pore diameters may range amounts of active catalyst into a reactor. More
from 3 nm to 500 μm, and the surface areas complicated shapes, often with fluted exteriors and
associated with them from <0.1 to >500 m2/gm. holes or channels in the body, allow enhanced
geometrical surface area which increases access to
In general, the higher the surface area, the greater the catalytic sites for the reactants.
the active materials’ dispersion and the more active
the catalyst. Note that maximum activity is not In environmental Figure 2: Catalyst Carriers
always desirable, as it may produce greater
quantities of less-preferred products. This may also, catalysis, pres-
with an exothermic reaction, cause temperatures to
rise resulting in shortened catalyst lifetime due to sure drop is often
coke deposition.
especially impor-
Pore diameter is also important -- large pores allow
easy access for large reactant molecules, whereas tant and the well-
the smaller pores contribute more to higher surface
areas. Bimodal or even trimodal pore size known honey-
distributions may be incorporated into a particular
carrier for a particular reaction system (Figure 1). comb shapes

Other important characteristics of the carrier are: used in automo-
material physical strength -- bed depths may be 40
bile catalysts are

used. Honey-

combs are also

being developed

for applications in

the chemical and petrochemical industries.

Figure 1: Pore Diameter Chemical Characteristics
The chemical properties of the carrier are also
important -- trace catalytic poisons must be avoided.
(The purity of the material, however, is often in direct
relationship with its cost). The pH and chemical
groups present on the surface can affect the catalyst
-- the material employed may be amphoteric, such as
alumina, or may involve other oxides such as silica
or zirconia. Silicon carbide may also be used where
thermal stability or conductivity is a priority.

The crystalline phase of the material often has
significant catalytic effects. Alumina, the most widely
used carrier material can exist in a number of
crystalline phases: from the gamma- phase present
in high surface area materials, through various

transition phases (delta-, theta-, eta-, etc.), to the 3
alpha- phase obtained by high temperature
calcination of alumina to very low surface areas. powders, adding peptizing and other agents to the
The source of the alumina and the manufacturing plastic mix, and incorporating organic solid particles
process affects the phases produced, and phase that are removed during the calcination stage
changes can be accelerated or retarded by the (burnouts). Manufacturing a ceramic material to
reaction conditions. Similar situations exist for titania meet these varying considerations is a complex
(anatase and rutile phases) and zirconia (tetragonal, operation (Figure 3).
monoclinic and cubic phases).
Figure 3: Catalyst Carrier Manufacturing Scheme
While alumina and silica remain the most common
carrier materials, titania, zirconia, silica carbide and Inorganic Materials Organic Materials
other materials are gaining significant share. (eg Alumina Powders) (eg Extrusion Aids, Burnouts)
Zirconia has very specific acid sites on the surface --
both Bronsted and Lewis acid activity may be Water
controlled by control of the zirconia phase.
Treatment of tetragonal zirconia to make sulfated or Mixing
tungstated zirconia yields highly acidic carriers --
sometimes called “superacids.” Other complex Shaping (Extrusion
metals/salts may also be formed and sintered in Drying Agglomeration, etc.)
similar ways to the oxides described here. Some
typical catalyst carriers, with their major physical and Calcination
chemical properties, are described in Table 2.

T ab le 2: T yp ical C arrie r P ro p e rtie s

Co m p o sitio n S urfa ce M e d ia n P o re Dia m e te r T o ta l P o re W a te r P a cking
Volum e A b so rp tio n
Are a 130 μm D e n sity
B IM O D A L ; 2 . 3 / 3 0 μm cc/gm %
m 2/gm TR IM O D A L ; 1 / 1 0 / 2 5 0 μm 27 kg/m 3
S A 5205 A LP H A -A LU M IN A / A M O R P H O U S S 0.02 B IM O D A L ; 2 5 / 5 5 0 n m 0.25 25 960
B IM O D A L ; 7 / 5 0 0 n m 0.53 50 1280
S A 5261 A LP H A -A LU M IN A 0.25 > 0.8 845
2.5 μm 1.05 28 550
S A 5262 A LP H A -A LU M IN A 0.75 15 nm 0.27 55 465
B IM O D A L ; 8 / 6 0 n m 0.38 1200
S A 3 11 3 2 M A IN L Y TH E TA -A L U M IN A 55 B IM O D A L ; < 3 / 2 2 0 nm 0.29 900
1.7 μm 0.37 1160
S A 6276 G A M M A -A LU M IN A 250 B IM O D A L ; 1 0 / 4 0 0 n m 0.23 1200
24 μm 1.2 825
XT 9 0 0 45 R U TIL E TITA N IA 0.33 0.43 350
560
S T 6 1 1 2 0 A N A TA S E TITA N IA 150

XZ 1 6 0 52 M O N O C L IN IC ZIR C O N IA > 85

S Z 3 1 1 5 2 TE TR A G O N A L ZIR C O N IA 200

S S 5 13 1 G L A S S Y S IL IC A 0.25

S S 6 11 3 8 S IL IC A 250

XC 6 9 3 7 4 S IL IC O N C A R B ID E / 1 6% S IL IC A 0.3

Zeolites are of great importance, especially in the oil Raw Materials
refining industry. These alumina-silica materials, Raw material choice is crucial, bearing in mind that
containing highly ordered microporous structures, raw materials from different sources can appear
are manufactured using sol gel technology. They are identical, but lead to differences in carrier
then formed into fixed- or fluid-bed catalyst carriers properties. If such differences in the carrier are
using the ceramic technologies described in this not seen, differences in catalytic performance can
article. New, zeolite-like, mesoporous materials, occur.
especially silicas, are showing considerable promise,
though their high cost may present a barrier to wide Tight control of raw material properties is a basic
industrial application. This may be alleviated by requirement. Most variations in finished carrier
coating the mesoporous material on a fibrous properties can be traced to variations in the
material, such as a quartz fiber mat. properties of the raw materials. Raw material
properties are charted before use so that any
Manufacturing Technology trends may be noticed -- close cooperation with
Porosity incorporation is achieved by three major raw material manufacturers is mandatory.
mechanisms -- partial sintering of carefully chosen

4

Each step of the manufacturing process is analyzed area, reduce the porosity, and increase the average
in detail so that predictive modeling can enable pore diameter.
finished product specifications be met. Mixing and
extrusion aids, as well as burnouts, can also add Additional Manufacturing Considerations
undesired chemical impurities. Fluid bed carriers are usually manufactured by spray
drying techniques, which in themselves are
Mixing extremely complex.
The raw materials are well mixed before extrusion
occurs. Mixing techniques can greatly affect finished The complexity of the overall manufacturing
product properties. Typically aqueous media are operation makes development and scale-up
used to form an extrusion paste which contains processes extremely important. All stages are
several base inorganic components (e.g. several monitored by statistical process control techniques.
aluminas) and organic mixing aids, extrusion aids,
rheology controllers and burnouts. Careful control of Finished catalysts are then manufactured, usually by
the plastic mix must be achieved by accurate impregnation of solutions of the metal salts required.
component weighing and tight control of mix times. In other cases, the catalytically active components
may be mixed with the raw materials and subjected
Shaping to the above processes to make coformed catalysts.
Shaping by extrusion is probably the most widely Even small quantities of additives can markedly
used technology. Extrusion may be by screw or affect all production steps, and therefore extensive
piston methods, with variations in design and experimentation and qualification is necessary if any
operation. Die design is crucial to the production of component of a formulation is to be changed.
consistent, well-shaped and strong particles.
Feedback to allow for drum-to-drum variations in raw Ceramic Bed Support Media
materials’ properties is essential because the nature A major application of ceramics in catalysis is the use
of the plastic extrusion mix affects, in particular, the of bed support and bed topping media. Figure 4
physical properties of the finished product. depicts a typical reactor loading scheme. These

Other bead and sphere formation Figure 4: Upflow Reactor
techniques may be used. Agglomeration
can occur on a cone or an inclined plane.
Proprietary technologies can achieve very
tight particle size control, similar to that
which can be achieved by employing gel
formation in a hydrophobic medium (the oil
drop method).

Drying and Calcination materials support the catalyst bed and also act as
Drying is carefully controlled as drying hold down layers.
rates, temperatures, airflows and humidities
are all important. Often called ceramic balls, their function as a bed
topping is to even out flow and temperature
Calcination, from temperatures of 400 - 1,500O C, is variations in the feed by providing good distribution,
the final stage. For lower temperatures, rotary such that the catalyst bed cross section sees a
calciners are common, usually indirectly fired. At uniform physical and chemical composition of
higher temperatures, both periodic and tunnel kilns reactant mix, thus avoiding uneven bed deactivation
are employed. The firing cycle must allow for the and premature breakthrough. This technology
release of volatile components, the removal of extends outside catalysis to desiccant, absorbent,
organic burnouts and partial sintering which and other applications.
induces the required pore size distribution. A
consistent temperature profile or program results in
a consistent carrier and thus consistent finished
catalyst properties. As a general rule, higher
calcination temperatures lower the internal surface

Figure 5: Denstone® Catalyst 5
Bed Support Media
to diffuse out enabling the material to withstand
extreme depressurization and temperature changes
(Table 4).

If high pressure steam is present, small quantities of
silica can leach out of the silica-alumina material and
deposit on downstream equipment. For such
applications, high-alumina (>99%) spheres are
employed.

Alumina-silica matrices are most often used, with High surface area alumina spheres are also used in
fired clay materials being most common. Desired active support media – materials impregnated with
physical properties include low porosity (to prevent catalytically active metals may allow the bed
unwanted catalytic activity), high strength, impact, supporting function with extra catalytic activity.
and abrasion resistance. Conditions at the top of the

bed may result in extreme variations in the velocity Special low pressure drop shapes, such as
and temperature of the incoming stream. pentarings, have been developed (Figure 6). This
material can also trap larger pieces of solid impurities
Spheres are often used, in several layers with that may be contained in the feed stream.
sizes from 2.5 - 50 mm. Consistent shape is
important in achieving well-controlled distribution. Figure 6: Pentaring Media
Typical properties are shown in Table 3 and a
commonly used material is depicted in Figure 5.

Advances in the development of bed support media
can overcome some traditional material
disadvantages such as support fracture, especially
under rapid depressurization conditions. Patented
materials have been developed with a uniform
microstructure that allows gas (especially hydrogen)

Ta ble 4: The rm a l Stre ss Prope rtie s of Be d Support Me dia

De nstone 2000 Tra ditiona l Silica -Alum ina

% Survival Crush Strength (kg)

Autocla ve Te st* 100 25 - 88

The rm a l Shock Te st** 207 33 - 94

* Depressurization to ambient from 450OC 100bar in hydrogen in 1 second

** Spheres at 425OC dropped into water at 10OC

6

Guard Beds for Particulate Removal Figure 8: MacroTrap® Guard Bed Media
Highly macroporous (pore diameters 50 - 500
μm) ceramic media may replace some of the Before (left) After (right)
bed topping layers of ceramic media and act as
a kind of filter. In many processes fine Over 100 installations worldwide have been
particulates, especially rust, (often known as successful, mainly in hydrotreating processes, but
“tramp iron”) may be trapped by the catalyst with increasing application in other chemical and
bed causing it to clog, with resultant increase in petrochemical processes. Again, a high alumina
pressure drop over the system. This can result version is available in applications where steam may
in premature replacement of the top layers of be present.
catalyst (“skimming”), a process that may result
in the closure of the reactor for 3 - 5 days, with Figure 9: MacroTrap® Guard Bed Media
consequent loss of production and revenue. Operational Performance

One such material, MacroTrap® media is depicted in Conclusion
Figure 7. Ceramics play a central role in catalysis and
therefore a central role in the production of many of
The dust particles are physically trapped in the the materials essential to modern-day life.
macropores and so do not significantly contribute to
any pressure drop increase. Electron microprobe
studies show the particles trapped in the
interconnecting pores. Polymer formation
may also occur in the pores and thus not
on the catalyst bed. Figure 8 depicts a
typical “before and after” situation.

Figure 9 shows how installation of a layer
of MacroTrap material in a desulfurization
unit allowed over 45 months’ operation
with no measurable increase in pressure
drop, compared to previous experience
which saw pressure drop increase by over
25 times in 25 months.

Figure 7: MacroTrap®
Guard Bed Media

www.norpro.saint-gobain.com © Copyrighted 2005 Saint-Gobain NorPro, 11/05